Leaverites - Features in Sedimentary Rocks

Ripple Marks

Ripple marks are undulating surfaces on bedding planes caused by waves or currents while
the rocks were being deposited.

Here are ripple marks on the muddy bottom of Green Bay, formed
during a low stand of the Bay.

Virtually identical ripple marks from the Baraboo
Quartzite, 1600 million years old.

Below are more ripple marks from
the same area.

When waves come from two directions at once,
interference ripple marks like these in Ontario form.

Modern interference ripple marks on the bottom of
Green Bay.

One way interference ripple marks can form.

Mud Cracks

Polygonal patterns resulting from the sediment drying and shrinking
before it was buried. These often have rims of whatever filled the cracks.

Mud cracks on the bottom of a modern puddle.

Mud cracks in the dolomite at Maribel Caves, Wisconsin, evidence
that the limy mud that formed these rocks occasionally
dried out. The rocks probably formed in a tidal flat
environment.

Mud cracks in Cambrian rocks in southwestern Virginia.
Even the curls of the flaking mud are preserved. We are
looking at the top of the beds here.

Cross-Beds

Usually seen on surfaces that break through the rock layers. Within the
layer are smaller slanting layers caused by shifting patterns of erosion and
deposition. Wind and water action can both produce cross-bedding.

These large cross beds probably formed in an offshore sandbar

Small cross-beds like these, from Scotland, are often called
"festoon" crossbeds. They formed from rapid cutting and infilling of
small channels, probably in an alluvial fan.

These large and fairly uniform crossbeds from southern Illinois
probably were made by a small stream delta during a marine invasion.
Shale interbeds represent quiet intervals when the delta was somewhere
else, then, later on the delta returned to this location when the water
was deeper. Three separate episodes can be seen. In each case the delta
built from right to left.

Cross-beds are rarely seen from above, but when they are they look
like this. Small channels were cut and then filled in from right to
left.

Huge cross beds like these are usually dune sands. You can't see
from the picture alone that they even are cross beds. Only by
noting that all the rock layers around are horizontal can you tell.

This rock contains several sets of cross beds nicely outlined by the
green mineral glauconite.

Graded Bedding

Layers where coarse material grades into finer material toward the top,
usually caused by fast-moving water that slows down. Floods and submarine
landslides are common causes.

Salt Crystal Casts

Relatively rare. These are square or angular impressions left by salt or
gypsum crystals as sediment dried out. The small square lumps on the rock at
right are salt crystal casts.

A closer view.

Reduction Spots

Reduction spots are circular (actually spherical) pale spots in
reddish sedimentary rocks. Something at the center causes the iron in
the surrounding rock to be less oxidized (or reduced). The
culprit can be a fossil or small mineral fragment.

Omars

Omar is short for Omarolluk. The Omarolluk Formation occurs in the
Belcher Islands of Hudson's Bay. Round calcareous concretions occur in a
fine-grained metamorphosed siltstone. When they weather out, they leave
hemispherical holes. These rocks are only known to occur in a small area
in Hudson's Bay but the glaciers spread them across central Canada and
into the northern U.S.

Here's a very large concretion that hasn't completely weathered out
yet.

Wave Action

This looks like horrible pollution but is actually
perfectly natural. On the Lake Michigan shoreline, wave
action washes away light minerals, leaving only black
magnetite sand at the high-water mark.

When wave conditions are right, layers of almost pure
magnetite can form, like those seen in this shallow
trench.

Load Casts

When sediment is deposited faster than it can compact,
it can sag downward into still-mushy layers below, and
mud can squirt upward. Sags like these are called load
casts. These are from the Antarctic Peninsula. Note
that the coarse bed is graded and the largest grains fill
the bottoms of the sags.

Flame Structure

Sometimes mushy sediment will find only a few weak
spots, and produce isolated structures like this, called
a flame structure. This one is near Sudbury,
Ontario.

The extremely smooth, flat surface is due to glacial action.

Sandstone Dike

This looks like a dike, but close inspection shows
that the rock is sandstone. A sandstone dike? Yes,
that's exactly what it's called. This one is north of
Lake Huron in Ontario. Sandstone dikes form when sediment
is partially consolidated but under high pressure. If a
water-laden layer can find a weak spot in the overlying
layers, it squirts upward. Earthquakes are a common
trigger. Note how large pieces concentrated in the center
where flow was fastest.

Soft-Sediment Deformation

The very uniform layers here are varves,
alternating thick and thin layers formed in a glacial
lake. The contorted zone running across the center is
probably due to glacial ice shoving the upper layers of
sediment over the lower layers. Deformation that occurs
while sediments are partly or wholly unconsolidated is
called Soft-Sediment Deformation

Sometimes a stack of layers will slip over a weak layer beneath,
rumpling like a carpet.

Flute Casts

Submarine landslides are common on the edge of the
continental shelf. The landslide flows scour flutes in
the underlying sediment, then come to rest as graded beds
of sand and silt that fill in the sculpted bottom. We are
looking at the underside of a submarine landslide deposit
in New Brunswick.

Looking at the underside of another submarine
landslide deposit in New Brunswick. The flutes scoured by
submarine landslides tend to be steep on the upstream end
and gently-tapered on the downstream end, making it easy
to tell which way they moved. These structures are hard
to observe directly forming in nature but quite easy to
duplicate in the laboratory.